Communication Between Applications

Applications can communicate with each other by writing to and reading from a
field in a specified call's information record, the LINECALLINFO data structure. With lineSetAppSpecific, any owner application can write to the application-specific field called dwAppSpecific. It is uninterpreted by the Telephony API or any of its service providers.
This field's usage is entirely controlled by applications.

The field can be read from the LINECALLINFO record returned by lineGetCallInfo. However, lineSetAppSpecific must be used to set the field so that changes become visible to other
applications. When this field is changed, all other applications with call handles are
sent a LINE_CALLINFO message with an indication that the dwAppSpecific field has changed.